Verified this in 6.4 it works fine for me <image    jcr:primaryType="nt:unstructured"    sling:resourceType="granite/ui/components/coral/foundation/form/pathbrowser"    fieldDescription="Image"    fieldLabel="Image Path [Desktop]"    name="./image"    rootPath="/content/dam"    value="/content/dam/w...

Yes its possible to set a default value for the pathbrowser field. value property should work fine to set a default value.